Conventional articles of athletic footwear include two primary elements, an upper and a sole structure. The upper is generally formed from a plurality of elements, such as textiles, foam, leather, and synthetic leather materials, that are stitched or adhesively bonded together to form an interior void for securely and comfortably receiving a foot. The sole structure incorporates multiple layers that are conventionally referred to as an insole, a midsole, and an outsole. The insole is a thin, compressible member located within the void of the upper and adjacent to a plantar (i.e., lower) surface of the foot to enhance comfort. The midsole is secured to the upper and forms a middle layer of the sole structure that attenuates ground reaction forces during walking, running, or other ambulatory activities. The outsole forms a ground-contacting element of the footwear and is usually fashioned from a durable and wear-resistant rubber material that includes texturing to impart traction.
The primary material forming many conventional midsoles is a polymer foam, such as polyurethane or ethylvinylacetate. In some articles of footwear, the midsole may also incorporate a sealed and fluid-filled chamber that increases durability of the footwear and enhances ground reaction force attenuation of the sole structure. In some footwear configurations, the fluid-filled chamber may be at least partially encapsulated within the polymer foam, as in U.S. Pat. No. 5,755,001 to Potter, et al., U.S. Pat. No. 6,837,951 to Rapaport, and U.S. Pat. No. 7,132,032 to Tawney, et al. In other footwear configurations, the fluid-filled chamber may substantially replace the polymer foam, as in U.S. Pat. No. 7,086,180 to Dojan, et al.
As an alternative to chambers, a footwear sole structure may also incorporate a fluid system that includes various components, such as a pressure chamber, a pump chamber for increasing a fluid pressure within the pressure chamber, one or more valves for regulating the direction and rate of fluid flow, and conduits that connect the various fluid system components. U.S. Pat. No. 6,457,262 to Swigart discloses a fluid system having a central chamber and two side chambers positioned on medial and lateral sides of the central chamber. Each of the side chambers are in fluid communication with the central chamber through at least one conduit that includes a valve. During walking or running, fluid within the fluid system may flow between the central chamber and the side chambers.
Fluid systems may also utilize ambient air (i.e., air drawn in from an exterior of the footwear or an exterior of the fluid system) as the system fluid. As an example, U.S. Pat. No. 6,889,451 to Passke, et al. discloses an article of footwear having a fluid system that utilizes ambient air to pressurize a pressure chamber. The fluid is drawn in through a filter, pressurized within a pump chamber in a forefoot area of the footwear, and transferred to a pressure chamber in a heel area of the footwear. When sufficiently pressurized, the pressure chamber serves to attenuate ground reaction forces. Another example of a fluid system utilizing ambient air is disclosed in U.S. Pat. No. 7,051,456 to Swigart, et al.